U.S. patent number 6,038,359 [Application Number 09/108,128] was granted by the patent office on 2000-03-14 for mode-routed fiber-optic add-drop filter.
This patent grant is currently assigned to Intelligent Fiber Optic Systems. Invention is credited to Richard James Black, Behzad Moslehi, Herbert John Shaw.
United States Patent |
6,038,359 |
Moslehi , et al. |
March 14, 2000 |
Mode-routed fiber-optic add-drop filter
Abstract
New elements mode-converting two-mode grating and mode-filtering
two-mode coupler are disclosed and used as elements in a system for
communications, add-drop filtering, and strain sensing. Methods of
fabrication for these new two-mode gratings and mode-filtering
two-mode couplers are also disclosed.
Inventors: |
Moslehi; Behzad (Mountain View,
CA), Black; Richard James (Palo Alto, CA), Shaw; Herbert
John (Stanford, CA) |
Assignee: |
Intelligent Fiber Optic Systems
(MT. View, CA)
|
Family
ID: |
25211635 |
Appl.
No.: |
09/108,128 |
Filed: |
June 4, 1998 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
813165 |
Mar 7, 1997 |
5940556 |
Aug 17, 1999 |
|
|
Current U.S.
Class: |
385/42; 385/24;
385/27; 385/28; 385/37 |
Current CPC
Class: |
G02B
6/2932 (20130101); G02B 6/29355 (20130101); G02B
6/29383 (20130101); G02B 6/14 (20130101); G02B
6/29334 (20130101) |
Current International
Class: |
G02B
6/34 (20060101); G02B 6/14 (20060101); G02B
006/26 () |
Field of
Search: |
;385/42,27,24,28,29,30,31,32,37,38,39 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Palmer; Phan T. H.
Attorney, Agent or Firm: Chesavage; Jay A.
Government Interests
This invention was made with government support under contract
NAS1-20579 awarded by NASA. The government has certain rights in
this invention.
Parent Case Text
This application is a division of application Ser. No. 08/813,165
filed on Mar. 7, 1997, now U.S. Pat. No. 5,940,556, issued Aug. 17,
1999.
Claims
We claim:
1. An optical mode-converting add-drop wavelength filter
comprising:
a two-mode short-period grating having a single port for input and
output, said grating converting fundamental-mode wave energy into
reflected second-mode wave energy at a specific wavelength;
a two-mode directional optical coupler having an input port and two
output ports, the first said output port responsive to wave energy
from said input port and coupled to said two-mode short-period
grating, and the second said output port responsive preferentially
to second-mode energy reflected from said first output port.
2. The mode converter of claim 1 wherein said two-mode short-period
grating is a two-mode Bragg grating fabricated in the core of an
optical fiber.
3. The mode converter of claim 1 wherein said two-mode short-period
grating is an external grating fabricated outside the core of an
optical fiber.
4. The mode converter of claim 2 wherein said two-mode optical
coupler is fabricated from two optical fibers, said optical fibers
modified to enable maximum coupling of second-mode wave energy and
minimum coupling of fundamental-mode wave energy.
5. The mode converter of claim 4 wherein said modification
comprises heating and drawing said optical fibers.
6. The mode converter of claim 4 wherein said modification
comprises mechanical polishing of said optical fibers.
7. The mode converter of claim 4 wherein said modification
comprises chemical etching of said optical fibers.
8. An optical mode converter comprising:
a first optical fiber having an input end, a two-mode coupler
middle section, and a grating end, said grating end comprising a
plurality of refractive index modulations formed into said optical
fiber, each of said refractive index modulations being coplanar and
at an angle of between 1.degree. and 8.degree. with respect to
central axis of said first optical fiber grating end;
a second optical fiber having an output end and a two-mode coupler
section, said two-mode coupler section responsive to reflected
second-mode waves from said first optical fiber two-mode coupler
middle section, and said output end positioned to receive said
reflected second-mode waves therein.
9. The mode converter of claim 8 wherein said two-mode coupler
section comprises said first optical fiber and said second optical
fiber heated and drawn together to satisfy said requirement for
maximum coupling of reflected second-mode waves and minimum
coupling of fundamental-mode waves.
10. An optical mode-converting device comprising:
an optical fiber supporting both fundamental and second-mode waves
having a first port end, a grating middle and an optional second
port end, said grating middle comprising a plurality of
short-period refractive index modulations formed into said optical
fiber, each of said refractive index modulations being coplanar and
at an angle of between 1.degree. and 8.degree. with respect to
central axis of said optical fiber grating middle, wherein said
fundamental-mode waves furnished to said first port end are
converted into second-mode waves at a single wavelength and
reflected back to said first port end, and all other wavelengths
continue to said optional second port end without such mode
conversion.
11. The optical mode-converting device of claim 10 wherein said
refractive index modulations are coplanar and at an angle chosen to
maximize the power level of reflected second-mode waves.
12. An optical mode-converting add-drop wavelength filter having an
input, an output, an add port, and a drop port comprising:
a two-mode long-period grating having a first port and a second
port, said second port producing second-mode waves converted from
fundamental-mode waves furnished to said first port, said first
port coupled to said input;
a two-mode directional optical coupler having a source port, said
drop port, said add port, and said output port, said drop port
selectively responsive to second-mode waves furnished to said
source port, and said output port selectively responsive to
second-mode waves furnished to said add port, said source port
coupled to said two-mode long-period grating second port.
13. The mode converter of claim 12 wherein said two-mode
long-period grating is a two-mode Bragg grating fabricated in the
core of an optical fiber.
14. The mode converter of claim 12 wherein said two-mode
long-period grating is an external grating fabricated outside the
core of an optical fiber.
15. The mode converter of claim 13 wherein said two-mode optical
coupler is fabricated from two optical fibers, said optical fibers
modified to enable maximum coupling of second-mode wave energy and
minimum coupling of fundamental-mode wave energy.
16. The mode converter of claim 15 wherein said modification
comprises heating and drawing said optical fibers.
17. The mode converter of claim 15 wherein said modification
comprises mechanical polishing of said optical fibers.
18. The mode converter of claim 15 wherein said modification
comprises chemical etching of said optical fibers.
19. An optical mode converter comprising:
a first optical fiber having sequentially an input end, a
long-period grating middle section, a two-mode coupler middle
section, and an output end, said grating end comprising a plurality
of refractive index modulations formed into said optical fiber,
each of said refractive index modulations being coplanar and at an
angle of between 1.degree. and 8.degree. with respect to central
axis of said first optical fiber grating end;
a second optical fiber having sequentially an add port end, a
two-mode coupler section, and a drop port end, said first fiber
two-mode coupler section and said second fiber two-mode coupler
section placed in close proximity wherein second-mode waves from
said add port are coupled to said output port, and second-mode
waves converted from said grating middle section are coupled from
said long-period grating middle section to said drop port.
20. The mode converter of claim 19 wherein said two-mode coupler
section comprises said first optical fiber and said second optical
fiber heated and drawn together to satisfy said requirement for
maximum coupling of second-mode waves and minimum coupling of
fundamental-mode waves.
21. An optical mode-converting device comprising:
an optical fiber supporting both fundamental and second-mode waves
having a first port end, a grating middle and a second port end,
said grating middle comprising a plurality of long-period
refractive index modulations formed into said optical fiber, each
of said refractive index modulations being coplanar and at an angle
of between 1.degree. and 8.degree. with respect to central axis of
said optical fiber grating middle, wherein said fundamental-mode
waves furnished to said first port end are converted into
second-mode waves at a single wavelength and disposed to said
second port end, and all other wavelengths are disposed to said
second port end without such mode conversion.
22. The optical mode-converting device of claim 21 wherein said
refractive index modulations are coplanar and at an angle chosen to
maximize the power level of converted second-mode waves.
23. A mode-splitting two-mode directional optical coupler
comprising:
a first two-mode fiber having an input end, a coupled middle
section, and an output end;
a second two-mode fiber having an input end, a coupled middle
section, and an output end;
a second-mode wave coupling section formed by placing said first
fiber coupled middle section with said second fiber coupled middle
section and heating and drawing said second-mode wave coupling
section until second-mode wave energy applied to input end of said
first fiber is maximally coupled to said output end of second
fiber, but before fundamental-mode wave energy applied to said
first fiber input end begins to appear at said second fiber output
end;
said heating and drawing performed while a source of said
second-mode wave energy is applied to said input end of first
fiber, and a detector of said second-mode wave energy is applied to
said output end of second fiber.
24. A mode-splitting two-mode directional optical coupler
comprising:
a first two-mode fiber having an input end, a coupled middle
section, and an output end;
a second two-mode fiber having an input end, a coupled middle
section, and an output end;
a second-mode wave coupling section formed by placing said first
fiber coupled middle section with said second fiber coupled middle
section and heating and drawing said second-mode wave coupling
section until second-mode wave energy applied to input end of said
first fiber is maximally coupled to said output end of second
fiber, but before fundamental-mode wave energy applied to said
first fiber input end begins to appear at said second fiber output
end;
said heating and drawing performed while a source of said
second-mode wave energy is applied to said input end of said first
fiber, and a detector of said second-mode wave energy is applied to
said output end of said second fiber, and a detector of said
fundamental-mode wave energy is applied to said output end of said
first fiber.
25. The mode-splitting two-mode coupler of claims 23 or 24 wherein
said heated and drawn section is between 5 mm and 20 mm long.
26. The mode-splitting two-mode coupler of claims 23 or 24 wherein
said first two-mode fiber and said second two-mode fiber are
separated by a distance of 1 to 5 microns in said heated and drawn
section.
Description
FIELD OF THE INVENTION
The current invention applies to the field of optical waveguide,
particularly fiber-optic, filtering devices which involve
bi-directional conversion of energy between two optical waveguide
modes and mode-dependent routing of energy as well as allow the
adding and/or dropping of wavelength channels from an optical
waveguide bus.
BACKGROUND OF THE INVENTION
Modern optical fiber typically comprises an inner glass core
surrounded by a glass cladding and a protective plastic jacket.
Guidance of electromagnetic waves is achieved by the core having a
slightly higher index of refraction than the surrounding
cladding.
Electromagnetic waves that propagate in optical fibers may be
decomposed in terms of optical fiber modes. Modes can be either (a)
bound core modes which have the majority of their energy confined
to the vicinity of the core and can propagate over long distances,
or (b) cladding modes or radiation modes which are rapidly
attenuated. Optical fibers can be classified as single-mode,
two-mode, few-mode, or highly multimode depending on the number of
bound core modes that they support.
The number of modes increases with the guidance parameter V which
is proportional to the product of (a) the ratio of the core
diameter .phi..sub.co with respect to the wavelength .lambda. and
(b) the numerical aperture NA which is related to the difference
between the core and cladding refractive indices n.sub.co and
n.sub.cl respectively, i.e.,
where
Typical values for the core diameter are of order 10 .mu.m for
single-mode and two-mode or few-mode fiber operating at
communications wavelengths of 1300-1550 nm, and 50 .mu.m or 62.5
.mu.m for highly multimode fiber. Whether single-mode or multimode,
the cladding diameter has most commonly an overall diameter of 125
.mu.m, and a plastic jacket diameter is typically 250 .mu.m for
standard fiber. The glass core is generally doped with germanium to
achieve a slightly higher index of refraction than the surrounding
cladding by a factor of roughly 1.001. The jacket is generally
plastic and is used to protect the core and cladding elements. It
also presents an optically discontinuous interface to the cladding
thereby preventing coupling modes in the cladding to other adjacent
fibers, and usually plays no significant part in the optical
behavior of the individual fiber other than the usually rapid
attenuation of cladding modes in comparison with bound core
modes.
Two-mode fibers have core dimensions of the same order as those for
single-mode fibers except that overall the guidance parameter V is
slightly larger, e.g., for fibers with a uniform core and cladding
indices (known as step index fibers), V is less than 2.4 for
single-mode fibers and between 2.4 and 3.8 for two-mode fibers.
Note that as well as or use a fiber which is designed to be
single-mode at typical telecommunications wavelengths of 1300 nm
and 1550 nm will function as two-mode at shorter wavelengths. One
can also fabricate fiber with a slightly larger core diameter
and/or NA to function as two-mode fiber at the above
wavelengths.
As described in the book by Snyder and Love entitled Optical
Waveguide Theory published by Chapman and Hall (London, 1983),
under the assumptions of longitudinal invariance and small index
differences for which the scalar wave equation is applicable, the
modal field magnitudes may be written
where
.beta. is the propagation constant
.omega. is the frequency
t is time
z is the axial distance
r,.phi. is the polar trans-axial position along the fiber.
Single-mode fibers support just one order of bound mode known as
the fundamental-mode which we denote as .PSI..sub.01, and which is
often referred to in the literature as LP.sub.01. The transverse
field dependence for the fundamental-mode in the vicinity of the
core may be approximated by a gaussian function as
where r.sub.01 is the fundamental-mode spot size. Two-mode fibers
support two orders of mode. In addition to the fundamental-mode,
two-mode fibers support a second order of bound mode which we
denote as .PSI..sub.11, and which is often referred to in the
literature as LP.sub.11. The transverse field dependence of the
second order modes in the vicinity of the core may be approximated
as
where r.sub.11 is the second-mode spot size
f.sub.1 (.phi.) is the rotation of the pattern described by
f.sub.1 (.phi.)=cos(.phi.) or sin(.phi.),
and the other variables and constants are as described above. The
optical fields of second modes spread out further into the
cladding, and require fibers with a larger optical fiber core
diameter and/or core-cladding index of refraction difference to
reduce attenuative effects, compared to fundamental-mode waves,
which have less spread in their field patterns, and hence can
propagate in optical fibers with smaller core diameters and/or
core-cladding index of refraction differences.
While the above equations describe fundamental and second-mode
waves in their most common mathematical forms, it is clear to one
skilled in the art that other two-mode wave systems are available
for separation and aggregation on the basis of modal
characteristic, among which (a) the first two Transverse Electric
(or Transverse Magnetic) modes of planar waveguides commonly known
as TE.sub.0 and TE.sub.1 (or TM.sub.0 and TM.sub.1), (b) two
polarizations of a given order of mode such as (i) planar waveguide
modes TE.sub.0 and TM.sub.0, and the polarized optical fiber modes
known as LP.sub.01 .sup.x and LP.sub.01.sup.y, as well as (c) the
higher level modes of the waves described here and in the
publications and patents cited herein, all of which are
incorporated by reference.
Fiber optic filters are well known in the art, and may be
constructed using a combination of optical fiber and gratings.
Using fiber of the previously described type, there are several
techniques for creating fiber optic gratings. The earliest type of
fiber grating-based filters involved gratings external to the fiber
core, which were placed in the vicinity of the cladding as
described in the publication "A single mode fiber evanescent
grating reflector" by Sorin and Shaw in the Journal of Lightwave
Technology LT-3:1041-1045 (1985), and in the U.S. patents by Sorin
et al U.S. Pat. No. 4,986,624, Schmadel et al U.S. Pat. No.
4,268,116, and Ishikawa et al U.S. Pat. No. 4,622,663. All of these
disclose periodic gratings which operate in the evanescent cladding
area proximal to the core of the fiber, yet maintain a separation
from the core. A second class of filters involve internal gratings
fabricated within the optical fiber itself. One technique involves
the creation of an in-fiber grating through the introduction of
modulations of core refractive index, wherein these modulations are
placed along periodic spatial intervals for the duration of the
filter. In-core fiber gratings were discovered by Hill et al and
published as "Photosensitivity in optical fiber waveguides:
Application to reflected filter fabrication" in Applied Physics
Letters 32:647-649 (1978). These gratings were written internally
by interfering two counter propagating electromagnetic waves within
the fiber core, one of which was produced from reflection of the
first from the fiber endface. However, in-core gratings remained a
curiosity until the work of Meltz et al in the late 1980s, who
showed how to write them externally by the split-interferometer
method involving side-illumination of the fiber core by two
interfering beams produced by a laser as described in the
publication "Formation of Bragg gratings in optical fibers by a
transverse holographic method" in Optics Letters 14:823-825 (1989).
U.S. Pat. Nos. Digiovanni 5,237,576 and Glenn 5,048,913, also
disclose Bragg gratings, a class of grating for which the grating
structure comprises a periodic modulation of the index of
refraction over the extent of the grating. Within this class of
in-fiber gratings, most of the art is directed to in-fiber gratings
having the Bragg plane of refractive index modulation perpendicular
to the principal axis of the core of the fiber optic cable. A new
class of grating involves in-fiber gratings with an angular offset
in the plane of refractive index modulation. This type of angled
grating is referred to as a mode-converting two-mode grating, and,
with properly chosen angle, has the property of converting
fundamental-mode power into second-mode power and visa versa.
Whether internal or external, both types of gratings can be
fabricated as short-period gratings, or long-period gratings.
Short-period gratings reflect the filtered wavelength into a
counter-propagating mode, and, for silica based optical fibers,
have refractive index modulations with periodicity on the order of
a third of the wavelength being filtered. Long-period gratings have
this modulation period much longer than the filtered wavelength,
and convert the energy of one mode into another mode propagating in
the same direction, i.e., a co-propagating mode, as described in
the publication "Efficient mode conversion in telecommunication
fibre using externally written gratings" by Hill et al in
Electronics Letters 26:1270-1272 (1990). The grating comprises a
periodic variation in the index of refraction in the principal axis
of the core of the fiber, such variation comprising a modulation on
the order of 0.1% of the refractive index of the core, and having a
period associated with either short or long-period gratings, as
will be described later.
Fiber-optic add-drop filters are a class of filter of particular
interest in multi-wavelength communications and sensor systems, and
are used for adding a wavelength channel to or dropping a
wavelength channel from an optical fiber bus carrying signals
consisting of multiple wavelength channels.
Optical fiber couplers are well known in the art, and generally
comprise two fibers as described above having their jackets removed
and bonded together with claddings reduced so as to place the fiber
cores in close axial proximity such that energy from the core of
one fiber couples into the core of the adjacent fiber. There are
currently two main ways of practicing this coupling, as well as a
third less-used technique. The first method is the side-polished
coupler, wherein the cladding material from each fiber is removed
through a mechanical polishing operation, followed by a bonding of
the two polished claddings together to allow evanescent coupling
between the fiber cores. Generally, these couplers are fabricated
from a pair of single-mode, or a pair of multi-mode fibers. The
side-polished class of fiber optic coupler is described in
publications "Single-mode Fibre Optic Directional Coupler" by
Bergh, Kotler, and Shaw in Electronics Letters, 16(7)(1980), and
"Determination of Single-mode Fiber Coupler Design Parameters from
Loss Measurement" by Leminger and Zengerle in the IEEE Journal of
Lightwave Technology, LT-3:864-867 (1985). A new class of
side-polished mode-converting couplers is described in "Highly
selective evanescent modal filter for two-mode optical fibers" by
Sorin, Kim and Shaw in Optics Letters 11:581-583 (1986). This class
of coupler is fabricated by polishing and bonding a single-mode
fiber with a two-mode fiber. As will be described later, this
mode-converting coupler converts fundamental-mode waves in a
single-mode fiber into second-mode waves, which are principally
coupled into the two-mode fiber. A second method of fabricating
optical couplers is a fused tapered coupler wherein the two fibers
are placed in close proximity, heated, and drawn together. The
fused tapered class of coupler is described by Hill et al in
"Optical fiber directional couplers: biconical taper technology and
device applications", Proceedings SPIE 574:92-99 (1985) with
analysis of their operation given in Bures, Lapierre, Lacroix
"Analyse d'un coupleur bidirectionnel a fibres optiques monomodes
fusionnees" in Applied Optics 22:1918-1921 (1983).
The third method of making couplers involves etching the cladding
as described in Single-mode power divider: encapsulated etching
techniques by Sheen and Giallorenzi in Optics Letters 4(1):29-31
(1979). Because of reciprocity, optical couplers fabricated from
single-mode fiber are intrinsically power-splitting reciprocal
devices. The most commonly used coupler involves two coupled
single-mode fibers and thus is intrinsically a 4 port device. If
such a coupler is used to extract the wavelength band reflected by
a single-mode grating, then, because of splitting-loss for the two
traversals of non-mode-converting coupler (before and after
reflection by the grating), a maximum peak power that can be
extracted is 25% of the peak power that would be reflected without
the coupler in the system. This least loss case involving
approximately 6 dB loss is for a 50/50 splitter known as a 3dB
coupler. Cascaded couplers of this type are frequently used in
single-mode systems, and the losses can become quite high, and
increase for each optical coupling event, as computed for one such
system in the publication "Analysis of the reflective-matched fiber
Bragg grating sensing interrogation scheme" by Ribeiro et al in
Applied Optics 36:934-939 (1997).
SUMMARY OF THE INVENTION
The present invention is directed towards a new class of filtering
devices which involves bidirectional conversion between two optical
waveguide modes as well as mode-dependent routing to (a) circumvent
the splitting loss problem associated with coupling reflected
energy out of single-mode waveguides using single-mode couplers,
(b) allow efficient addition/dropping of wavelength channels
to/from a primary waveguide from/to a secondary waveguide, and (c)
provide a basis for modal processing and modal logic systems. The
invention is illustrated with respect to the fundamental and
second-mode waves in optical fibers using optical elements
comprising optical gratings and optical couplers, all fabricated
using optical fiber. A first objective of the invention is low loss
conversion from the second-mode to the fundamental-mode. A second
objective of the invention is low loss conversion from the
fundamental-mode to the second-mode. A third objective is the
efficient wavelength-dependent extraction of the converted mode
into a second fiber using a mode-splitting two-mode coupler to
provide the basis for an add-drop filter. A fourth objective is the
creation of a wavelength-division multiplexed optical system.
Objectives one and two can be achieved with mode-converting
two-mode gratings. Methods of realizing objectives three and four
include (a) in an all-two-mode-fiber system or subsystem, combining
a mode-splitting two-mode coupler with a mode-converting two-mode
grating, and (b) in a single-mode-fiber system or subsystem, adding
a two-mode-fiber tapping section with a mode-converting grating
between two mode-converting single-to-two-mode couplers.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a is the plan view of a prior art side polished fiber
coupler.
FIG. 1b is the plan view of a prior art fused tapered fiber
coupler.
FIG. 1c is a section view of a prior art optical coupler.
FIG. 2 is an in-fiber two-mode grating.
FIGS. 3a-3c shows power transfer curves for a mode-converting
two-mode grating.
FIGS. 3d-3g shows modal wave energy patterns.
FIGS. 4a-b are the mode-converting wavelength-selective
filters.
FIG. 5 is the optical coupling characteristic of the mode-splitting
two-mode coupler.
FIG. 6 is a set of spectral density plots for power found at
various ports of the mode-converting wavelength-selective
filter.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to FIG. 1a, there is shown a prior art single-mode
optical coupler. For reference, we first examine the
characteristics of the single-mode fiber itself. Core 10 and
cladding 11 have respectively dimensions of approximately 10 .mu.m
and 125 .mu.m, and a ratio of refractive index in the range of
1.001. The protective plastic jacket is not shown as it is
typically removed during the process of fabricating the coupler.
The coupler is fabricated from a first fiber comprising a core 10
and cladding 11, which is placed adjacent to a second fiber also
having a core 12 and cladding 13. There are two methods commonly
used to create this adjacency. The first method is side polishing
wherein two fibers 11 and 13 are placed in a fixture and a
flattened surface is created through the polishing and reduction of
claddings 11 and 13, and the two side polished fibers are then
placed with side polished surfaces in contact with each other as
shown in FIG. 1a. Usually, fibers 11 and 13 are of the same type:
either both are single-mode or both are multimode. For a simple
single-mode coupler carrying fundamental-mode waves, the energy
transfer from a wave presented to port 10 and coupling to port 38
depends on length 15 and proximity 14, and can vary from 0% to
100%. For the case of a mode-converting coupler where core 10 is
single-mode fiber and core 12 is two-mode fiber, a fundamental-mode
wave presented at port 10 would convert into a second-mode wave
within the interface region 15, and in the ideal case, negligible
fundamental-mode wave energy would appear at port 37, and all of
the converted second-mode waves would be present at port 38.
Fundamental-mode wave energy presented to port 38 would not
mode-convert, and would appear unmodified at port 12. By duality,
second-mode wave energy presented at port 38 would convert to
fundamental-mode wave energy appearing at port 10.
The second method of fabricating an optical coupler is to place two
fibers 16 and 17 together and heat and draw them as shown in FIG.
1b, thereby necking the claddings 16 and 17, and respective cores
25 and 39, and creating a region of adjacency 18. Most modern
couplers are fabricated through the process of heating and drawing
the first fiber 16 along with the second fiber 17, until the cross
section of the new fused section resembles FIG. 1c, which shows
fused claddings 16 and 17, and diameter-reduced cores 25 and 39.
Coupling ratios are controlled by both the length of core coupling
18 and core separation distance 14. Typical values vary widely, but
may be found to be 10 mm for coupling length 15 and on the order of
one to several microns for distance 14. While the process of
fabricating a two-mode coupler has been described using fusing of
elements, it should be clear to one skilled in the art that any
other method of fabrication which places the fiber cores in
proximity within the cladding will produce the described two-mode
coupler, and such methods include etching, and many other
mechanical and chemical means. It has been observed that the
coupling function between the two fibers changes with drawing
distance and proximity.
For clarity, we will now define the three classes of couplers
relevant to this invention. Prior art optical couplers which couple
energy from the fundamental mode of a single-mode fiber to the
fundamental mode of another single-mode fiber will be referred to
as simple optical couplers. Prior art optical couplers which
convert from one mode to another will be referred to as
mode-converting couplers. The present invention is an optical
coupler which selectively couples energy of only one of the two
modes of two-mode fiber to the same mode of a second two-mode fiber
and will be referred to as mode-splitting two-mode coupler or
simply a two-mode coupler.
FIG. 2 shows an in-fiber grating. Core 20 is surrounded by cladding
21, and a modulated index of refraction region 22 is created in the
bulk of the core 20 through a variety of techniques, such as
exposure to a laser beam with periodically varying intensity along
the grating. In the case where angle 23 is 0.degree., the principal
effect of the grating is to reflect the fundamental mode into the
counter-propagating version of itself rather than the conversion of
modes. With regard to the period of the grating 22, there are two
distances in which the grating period may be set for a desired
filter function. For transmission of waves through the grating at
wavelength .lambda..sub.b, the long-period grating function is as
follows:
where
.LAMBDA..sub.b =pitch of the desired Bragg grating,
.lambda..sub.b =wavelength to reflect,
n.sub.1 =effective index of refraction of the first mode,
n.sub.2 =effective index of refraction of the second-mode.
In the case of a reflected wave returning to the entry port, a sign
reversal occurs for n.sub.2, wherein the formula becomes for the
short-period grating:
FIG. 3a is a diagram showing this filtering action for a
short-period grating and a fundamental-mode source. For a
spectrally flat excitation into input port 26, the resulting
transmitted wave energy spectrum is shown as curve 30, while the
reflected wave energy is shown as curve 31. It should be noted that
this type of filter is capable of very low transmissive and
reflective losses. As can be seen, for short-period gratings, the
overall behavior is that of a band pass filter for wave energy
reflecting back to the input port 26, and a notch filter for wave
energy passing on to the output port 27. As the grating is
longitudinally and axially symmetric, the labeling of input and
output ports is arbitrary, although it would be possible to
fabricate a grating without such symmetry to achieve other
filtering effects.
Examining now the effect of changing the angle of modulation, FIG.
3b shows the mode conversion property of the grating 22 as the
angle 23 is increased. Referring now to FIG. 3b, the effect of
angle .THETA. 23 is seen in the power reflected at the operating
wavelength .lambda.. For single-mode input excitation, curve 28
shows fundamental-mode power reflected as a function of grating
angle .THETA., while curve 29 shows second-mode power reflected as
a function of grating angle .THETA.. As can be seen from the
curves, at the experimentally determined grating angle .THETA.
36=1-8.degree., maximum transfer of power from fundamental-mode to
second-mode occurs. It is clear that the optimum angle for modal
power transfer will vary with different materials, but this is what
is observed for commercially available germanium doped silica glass
fiber. Given this modal conversion property of the fiber, it can be
seen that a fundamental-mode source will return a mixture of
fundamental-mode and second-mode energy. FIG. 3c shows this effect
for a short-period two-mode grating excited at port 26 by a
fundamental-mode source. Reflected power curve 33 now shows two
peaks. Reflected fundamental-mode power peak 35 remains at a
similar wavelength as from FIG. 3a, but a new reflected power peak
at wavelength 34 represents second-mode reflected power created by
the two-mode grating 22. Transmitted power curve 32 represents the
power transmitted to port 27, and as in the case of the single-mode
grating, minimal losses occur. FIG. 3d shows the optical fiber
cross section plot of the field magnitude of a fundamental-mode
wave pattern .PSI..sub.01, and FIG. 3e is the corresponding
amplitude plot for such a wave. FIG. 3f shows similarly the optical
fiber cross section plot of the second-mode wave amplitude for the
.PSI..sub.11 wave pattern, and FIG. 3g is the wave magnitude
plot.
FIG. 4a discloses a key form of the present invention, comprising a
two-mode fiber first port 40, two-mode fiber 42 leading to
mode-splitting two-mode optical coupler 43, another length of
two-mode fiber 44 terminating in short-period mode-converting
two-mode grating 45. One port of two-mode coupler 43 is coupled to
the wavelength-selective port 41, which has a second-mode output
which is obtained by conversion by the grating of the
fundamental-mode optical energy from the input source 40. It will
be seen that if fundamental-mode wave energy is provided to first
port 40, in general, the power reflected by short-period grating 45
with arbitrary grating angle .THETA. 36 will be a mixture of
fundamental-mode and second-mode energy, and the second-mode energy
will selectively be coupled by two-mode coupler 43 into fiber 48,
and conducted to output port 41, which will be found to contain
mostly converted second-mode energy. Conversely, if second-mode
energy is provided at port 41, then it will be selectively coupled
across two-mode coupler 43 and directed along fiber 44 to
short-period two-mode grating 45, wherein the grating will convert
such second-mode waves to fundamental-mode waves reflected back to
input port 40 via fiber 42. In accordance with the low coupling of
fundamental-mode waves across two-mode coupler 43, virtually all of
the fundamental-mode wave power reflected by grating 45 will be
sent back to port 40. However, in the case where two-mode grating
45 optimized for maximum reflection of second-mode waves compared
to fundamental-mode waves, port 46 is an auxiliary port for
measurement, and will contain wave energy not reflected by grating
45. FIG. 4b illustrates this same filter device fabricated using
long-period mode-converting grating 151. Fundamental-mode waves
from source 150 travel through fiber 154, and waves at
mode-converting wavelength .lambda..sub.n are converted from
fundamental-mode to second-mode waves but continue to propagate
towards two-mode coupler 152. Waves not at mode-converting
wavelength .lambda..sub.n continue to two-mode coupler 152 without
any mode change. Two-mode coupler 152 then couples second-mode
waves at wavelength .lambda..sub.n into drop port 157, while
non-mode converted fundamental-mode waves continue along fiber 155
to port 153. Second-mode waves furnished to add port 158 couple to
fiber 155 in two-mode coupler 152 and travel on to output port
153.
An important feature of the optical two-mode coupler is shown in
FIG. 5. It was disclosed earlier that one method of fabricating
optical couplers involved placing them in proximity to each other
and drawing them under elevated temperature. FIG. 5 shows the
fundamental-mode transfer function 50 through a first fiber as a
function of drawing distance .DELTA.l. Also shown is the
second-mode transfer function 51 for the same fiber drawn distance
.DELTA.l. The critical point l.sub.opt 52 shows the optimum drawing
distance where the second-mode energy reflected by the grating 45
and traveling back to the source 40 in fiber 44 is selectively
coupled such that the second-mode wave coupling to fiber 49 is at a
maximum, while the second-mode energy remaining in fiber 42 and
returning to the source 40 is at a minimum, while virtually all of
the fundamental-mode energy continues from 44 to 42 without
appreciably coupling to 49. The effect of stopping the drawing
process at this critical point creates a mode filter acting on the
reflected second-mode energy which passes with minimal attenuation
fundamental-mode wave energy, while filtering second-mode wave
energy onto the second fiber 49.
In practice, fabrication of the add-drop filter requires all
elements of FIG. 4a or 4b to be present, and while port 40 or 150
is illuminated with a fundamental-mode source, the two-mode fibers
47 and 48 comprising the two-mode coupler 43 or 152 are heated and
drawn while power is measured at port 41 or 157. At first, no
appreciable power is measured at port 41 or 157, and as the
claddings begin to fuse, some coupling of power is observed, and as
the drawing process begins, a point is reached where second-mode
power is maximally coupled to port 41 or 157. At this point, the
drawing process stops, as point 52 has been identified, and the
add-drop filter is now optimized.
FIG. 6a shows the typical power density spectrum 60 of a broadband
fundamental-mode source such as an LED (light emitting diode),
which is coupled into port 40. FIG. 6b shows the typical power
density spectrum 63 measured at 41 before fusing of element 43
begins, and also shows the progression in spectral density 63a, 63b
during fusing and drawing. As more wave energy couples into fiber
48, curves 63a and 63b begin to reflect this increased power level.
Curve 65 in FIG. 6c shows the final power density spectrum at the
critical coupling point 52, and is the point at which forming of
two-mode coupler 43 is terminated. Curve 64 shows the power
delivered to port 46.
* * * * *